A method for increasing the speed of color conversion during the printing of pages containing images in different color spaces by encoding to reduce the number of pixels by a factor of two prior to the color space conversion step.
In a printing system having low resolution, the boundaries between text letters or computer generated graphics and the background color may be jagged. This is called "aliasing". On the other hand, contone pictures usually have softer outlines, and aliasing is not a problem. For this reason, text and graphics are frequently supplied at higher resolutions than contone.
When combining text and contone in a single page to be printed, the two different pixel densities, when they occur, need to be equalized, and to preserve image quality, it is the contone pixel density that is typically increased up to the density of the text pixel density. Thus, for example, if the contone has a density of A by B pixels per unit area and the text has a density of 2A by 2B pixels, the contone pixels will be increased up to 2A by 2B, by duplicating pixels, before being combined. The resultant picture is not always optimal, so some kind of filtering or image enhancement is usually applied at this stage.
Now the page can be printed, but it frequently happens that the pixel density that the printer can print is less than the density that is being processed, and so the pixel density must be reduced. To continue with the above example, the page, which has been assembled into a 2A by 2B set of pixels must now be printed at a density of A by B. The fundamental challenge in accomplishing this reduction is to minimize any losses in image quality and to prevent aliasing from re-occurring. A common process for doing this is shown in FIG. 1. First, the image 10 can be divided into aligned 2-by-2 blocks of "subpixels"11, where an aligned block contains four pixels of contone which originated from one original contone pixel, or contains four pixels of text or graphics or background. These are averaged to compute a new "full pixel"12 which can be printed as the final image 13.
In the situation where the printer can print in full color,--assume CYMK,--there must be a color conversion step to convert all of the color images into one common color space. This is necessary since pixels of different color spaces can not be averaged. Thus, for example, if some of the areas within the image 10 are RGB and some CYMK, then all images scanned into the system in RGB must be converted into CYMK before the subpixels of FIG. 1 can be averaged. This color conversion step, which must be accomplished before the averaging step at the high-density subpixel level, is time consuming because of the larger number of subpixels at the higher density, and will slow down the system throughput if not compensated for. What is required is a faster color conversion process.